On October 6, 2011, scientists using the the
European Space Agency's
Venus
Express mission have detected a thin, tenuous layer of ozone high
in the planet's atmosphere like Earth and
Mars, in addition to the high layer of sulfur
dioxide detected in 2010. A molecule containing three oxygen atoms,
computer models suggest that ozone on Venus is formed when sunlight
breaks up carbon dioxide molecules and releases oxygen atoms, which
are swept around to the planet's nightside by winds where they can
then combine to form unstable, three-atom ozone molecules, as well as
much more stable, two-atom molecules essential for animals. Earth's
much thicker layer of low-level ozone, however, has a much larger
contribution from the build-up of molecular oxygen beginning some
2.4 billion years ago from photosynthetic microbes excreting oxygen
as a waste gas, which now along with plant life is constantly
replenishing Earth's two-atom as well as three-stom ozone oxygen
molecules. The presence of ozone helps to explain the detection
of hydroxyl (an unstable molecule of oxygen with a single atom of
hydrogen) high in planet's atmosphere in 2008 (ESA news releases on
ozone,
sulfur
dioxide, and
hydroxyl;
Lisa
Grossman, New Scientist, October 6, 2011; and
Montmessin
et al, 2011).

Alternative, larger and
jumbo
illustrations of hydroxyl in Venus'
nightside by ESA and C. Carreau.

A tenuous, thin layer of ozone
has been detected high in Venus'
atmosphere that probably forms
in its nightside
(more).

The Planet

The second planet from the Sun at about seven
tenths of the Earth-Sun distance, Venus is the sixth largest planet
but has no moon. It has the most circular orbit of all the planets
in the Solar System, with an eccentricity of less than one percent.
(See an animation of Venus' orbit
around the Sun, with a table of basic orbital and physical
characteristics.) However, its rotation is very slow with 243 Earth
days in a single "Venus day," which is slightly longer than the
planet's year, and retrograde when compared with Earth's so that the
Sun rises in the west and sets in the east. The planet's period of
rotation is synchronized with its orbital period so that
Earth always sees the same face when Venus is
at its closest approach. On the other hand, Venus can be seen from
Earth with the naked eye as the brightest "star" in the night sky.

At a meeting of the American Astronomical
Society on October 9, 2006, two planetary scientists (Alex Alemi
and David
Stevenson) described model results which suggest that Venus may
have once had a moon that was subsequently destroyed. Under current
theories of solar system formation, Venus is unlikely to have avoided
a protoplanetary collision large enough to create a moon, and computer
simulations suggest that most large collisions create a debris disk
from which a moon forms. Alemi and Stevenson suggest that a sequence
of two large collisions within around 10 million years could have
first created a moon but then destroyed it and sent debris crashing
onto Venus; this eventually created the planet's slow retrograde
rotation today, which is otherwise difficult to explain
(Alemi
and Stevenson, 2006; and George Musser, Scientific AmericanScience
News, October 10, 2006).

The planet was once thought to be a near twin of Earth because of its
similar size, being only slightly smaller with 95 percent of Earth's
diameter at 12,103 kilometers (7,522 miles) and 80 percent of its mass.
The densities and chemical compositions of the two rocky planets are
also similar, and radar observations found that Venus has relatively
few craters indicating a relatively young or quickly changing surface
like Earth. As a result, it was thought that below the planet's dense
clouds, Venus might even have life. However, more recent
investigations have found quite the opposite.

The planet is covered with thick white and yellowish clouds made of
sulfuric acid droplets, instead of water. There are strong winds of
350 kilometers per hour (218 miles per hour) at the cloud tops, but
surface winds are weak -- no more than a few kilometers (or miles) per
hour. Venus has no oceans and is surrounded by a heavy atmosphere
composed mostly of carbon dioxide.

The atmospheric pressure at the surface is equivalent to 92 atmospheres,
which is like the pressure found one kilometer (0.62 miles) down
Earth's oceans.

This dense atmosphere generated a
runaway
greenhouse effect that has raised the planet's surface temperature
by about 400 degrees Celsius to about 467° C (862 °F), which is hot
enough to melt lead. Sunlight passes through the atmosphere to heat the
surface of the planet, but this solar heat is trapped by the dense atmosphere
instead of radiating back out to space as efficiently as it does on Earth.
As a result, Venus' surface is actually hotter than
Mercury's despite being nearly twice as far
from the Sun.

A vortex cloud structure has
been found at the south pole,
mirroring a similar structure
at the north pole
(more).

On September 23, 2010, scientists using the the
European Space Agency's
Venus
Express mission released infrared images of a single storm vortex at
the planet's South Pole, although a double vortex was apparently detected
both by NASA's
Pioneer
Venus Mission in 1979 and the ESA's Venus Express mission
in 2006.
The single vortex extends over 3,000 kilometers (1,900 miles) in diameter,
and recently released videos (including
A
and B) of the
vortex suggest similarities with a
vortex over
Saturn's South Pole. More observations may
confirm whether a double vortex does sometimes develop over time
(Europlanet
press
release with more images and animations; and
Astronomy Picture of the Day).

During initial orbit capture on March 12, 2006, the
European Space Agency's
Venus
Express spacecraft took photos of sunlight
reflected from the tops of clouds down to a height of about 65
kilometers (40.4 miles) above the planet’s surface which revealed a
vortex cloud structure at the south pole, mirroring previous
findings at the north pole
(more).
On September 18, 2008, the ESA mission released an image, animation, and
illustration discussing the different atmospheric layers and respective
wind speeds between the equator and from 50 to 55 degrees latitude on the
planet's southern hemisphere, as measured between April 2006 and June 2007.
Using three different visual and infrared wavelengths to observe three
different atmospheric layers, Venus Express tracked the movement of clouds
and determined the speed of the winds moving them. Faster winds were
observed above the sulfuric acid clouds and haze layers, rather than below
(more
discussion).

Winds above the sulfuric acid
clouds and haze are faster than
those observed below
(more).

On February 21, 2008, the
European Space Agency's
Venus
Express released ultraviolet images that reveal the planet to
have large-scale weather changes. Rich in sulfuric acid, bright hazes
appear in a matter of days, reaching from the planet's south pole to
its low southern latitudes, and disappear just as quickly. Such hazes
are created when water vapor is lifted by an unknown process with sulfur
dioxide from lower levels into the planet' upper atmosphere, where it
is broken down by sunlight into molecules that recombine into volatile
sulfuric acid.
Under ultraviolet light examination, transient dark and bright stripes
mark Venus' atmosphere, indicating regions where Solar ultraviolet
radiation is either absorbed or reflected, respectively (more from
Venus
Express and
APOD).

Venus has large-scale weather
changes with bright, high-
altitude hazes that appear in
a matter of days, and disappear
just as quickly
(more).

Venus probably once had large amounts of water like Earth, but the
planet is now quite dry. Its oceans probably boiled away into its
atmosphere as its runaway greenhouse effect progressed. This may have
occurred because Venus is closer than Earth to the Sun, which has also
grown about 40 percent brighter over the past 4.6 billion years. One
recent theory suggests that Earth may suffer the same fate within 900
million years as Sol continues to brighten by about 10 percent.

Venus may have lost oceans of water
due to a runaway greenhouse effect
which evaporated water into the upper
atmosphere, where ultraviolet light
dissociated water into ionized atomic
hydrogen and oxygen (some later
incorporated into carbon dioxide)
that were blown away by the Solar
wind due to the lack of a strong
magnetic field like the Earth's
(more).

On November 28, 2007, the
Venus
Express mission released images and illustrations describing the
"true extent" and processes by which the Sun has been stripping away the
planet's atmosphere, including oceans of water over the past five billion
years (ESA
news
release -- more discussion below).
Venus is not protected by a strong magnetic field like Earth, and so
its upper atmosphere probably has been much more affected by the Sun's
Solar
wind, which blows away relatively light, hydrogen, helium, and
oxygen atoms much faster than happens in Earth's atmosphere. As a
result, once water molecules are dissociated into ionized
hydrogen and oxygen atoms by the Sun's ultraviolet light in Venus'
upper atomsphere, they are more easily blown into space by the
Solar wind
(S.I.
Rasool, 1968). Although many scientists believe that Venus
may once have had oceans of water on its surface (in part because
its
ratio of deuterium to ordinary hydrogen is now measured to
be around 150 times that of the Earth's), most of it has been
lost the past five billion years. (More discussion on the
processes
believed to be driving the evolution of Venus' atmosphere from
Professor Nick Strobel).

Around the
Venera 13
probe at bottom, the dissicated, daytime landscape of Venus displays
a yellowish-orange hue under a cloudy yellowish-white sky and the
natural, mustard-colored
light filtering through the planet's thick atmosphere

Around 1988, Jeffrey Kargel of the US Geological Survey estimated
that Venus may have lost its water around four billion years ago --
just 600 million years after the Solar System's birth. In 2003,
David Grinspoon of the Southwest Research Institute in Boulder,
Colorado suggested, however, that water may have persisted for
another two billion years. Kargel's estimate should be considered
to be a lower limit on when the planet dried up because it did not
include the effect of clouds in the Venusian atmosphere. The
clouds would have reflected sunlight back into space to cool
the Venusian surface, so that its atmosphere would have been 100
Kelvin cooler than without them.

Wider view of rocks around the base of the
Venera 13
probe,
which survived for only 127 minutes on Venus' surface.

On July 14, 2009, the
European Space Agency's
Venus
Express released infrared mapping data of the planet's southern
hemisphere, which suggest that Earth's neighbor once produced large masses
of granitic rock. On Earth, plate tectonics forces basaltic rock deep
closer to the hot mantle until melting and mixing with water forms lighter
granitic rock that eventually rises to the surface through volcanic activity
to form continental crust
(more).
As rocks on Venus'
Phoebe
Regio and
Alpha
Regio plateaus are slightly cooler and lighter in color, they
appear older than those found in most of the rest of the planetary
surface and so may be ancient continents that were created by
volcanoes in an ancient ocean.
(The ESA spacecraft arrived at Sol's hottest planet on March 11, 2006,
where it is being used to investigate how Venus -- although similar to
Earth in size, mass, and composition -- evolved over the past 4.6 billion
years to have atmospheric and planetary surface characteristics that now
appear very different from those on Earth.)

Cooler (bluer) plateaus in
Venus' southern atmosphere
may be composed of older
rocks produced by plate
tectonics with volcanoes
in an ancient ocean
(more).

Most of Venus' surface consists of gently rolling plains with little
relief. There are several broad depressions and two large highland
areas, one in the northern hemisphere about the size of Australia and
one along the equator about the size of South America. Radar
observations find that about 80 percent of the surface is covered
by lava flows with a few Australia-size uplands of two to five km
(1.2 to 3.1 miles) high, and there are large shield volcanoes that
rise above the plains as high as 11 km (6.8 miles) -- higher than
Mount Everest's eight km (five miles) above Earth sea-level.

Lava flows in the
Western
Eistla Regio
extend for hundreds of kilometers
across fractured plains to the base
of Gula Mons, a three-km (1.86-mile)
high volcano (upper left), and to
two-km (1.2-mile) high volcano Sif
Mons (upper right), which has a
diameter of 300 km (180 miles).

Venus may still volcanically active in a few hot spots, but in general,
it has been geologically quiet for the past few hundred million years.
The oldest surface features seem to be about 800 million years old,
but extensive volcanism about 300 to 500 million years ago appears to
have wiped out earlier surfaces including large craters from the
planet's early history. There are no small craters because small
meteoroids burn up in the dense atmosphere before reaching the ground,
and the bunching of craters suggest that large meteoroids usually break
up before reach the surface. Courtesy Jet Propulsion Laboratory. Copyright (c) California
Institute of Technology, Pasadena, CA. All rights reserved.

Around 700 million years ago, the entire Venusian surface appears to
have melted and reformed, which may have been a continuation of the
greenhouse warming that dried out the planet. According to David
Grinspoon, once Venus lost its water, plate tectonics would have
stopped and so the planet would lost the most efficient way for it
shed its internal heat. Eventually, the heat built up under the
crust until it reached melting temperatures, allowing heat to be
more quickly radiated into space.

On April 8, 2010, the European Space Agency's
Venus
Express mission has detected relatively recent lava flows around
what appear to be nine volcanoes on the surface of Venus.
The lava flows appear to have different compositions from surrounding
terrain which appear to be due to a lack of surface weathering. This
finding suggests that the flows erupted relatively recently, within 2.5
million years ago or even more recently. It is possible that the
volcanoes are currently active (ESA
new
release; and
Dennis
Overbye, New York Times, April 9, 2010).

Relatively fresh lava has
been detected around nine
volcanoes on Venus
(more).

The interior of Venus is probably very similar to that of Earth, with
an iron core about 6,000 km (3,700 miles) in diameter radius and a molten
rocky mantle comprising most of the planet. Gravity data indicate that
its crust is strong and thick. Convection in the mantle stresses the
surface like the Earth but is relieved in many relatively small regions
instead of being concentrated at plate boundaries as happens on Earth.
Venus has no magnetic field, perhaps because of its slow rotation.

David Seal (a mission planner and engineer at NASA's Jet
Propulsion Laboratory at CalTech) has a web site that generates simulated images of the Sun,
planets, and major moons from different perspectives and at different times of the year. Try
his Solar System Simulator.

For more information about the Solar System, go to William A. Arnett's
website on
"The Nine Planets",
or to Calvin J. Hamilton's web page on
"Venus".